The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Major to catastrophic lost circulation is a persistent and expensive problem in drilling and cementing. Polyurethane composition (PU) has been proposed repeatedly as a lost circulation material (LCM) over the last five decades—it is attractive for this use because the gelation time is under chemical control. In general, the chemistry of gelation starts when a polyalcohol/polyamine is blended with a polyisocyanate to give a fluid that will cure to a solid mass. In previous efforts, such fluids are either mixed with catalysts and/or retarders at surface prior to pumping, as shown in U.S. Pat. No. 3,181,611 and U.S. Pat. No. 3,181,612, both to Dollarhide, or formed downhole by some tool or trick of plumbing that mixes the polyalcohol and polyisocyanate in situ, as shown in U.S. Pat. No. 3,726,340 to Fraser. All references above are incorporated herein in their entirety, by reference thereto.
A PU LCM must have a pumpable rheology when uncured. After curing begins, the LCM should develop a rheology that will effectively sweep drill mud from the loss zone. It should achieve final cure in the loss zone. Failure to control the onset of gelation and the rate of gelation are the major causes of failure when PU LCMs have been applied.
In general, the process of PU gelation starts when a polyalcohol/polyamine is blended with a polyisocyanate to give a fluid that will cure to a solid mass. In previous efforts, such fluids are either mixed with catalysts and/or retarders at surface prior to pumping, or formed downhole by some tool or trick of plumbing that mixes the polyalcohol and polyisocyanate in situ. Since knowledge of the transit time and/or temperature profile the fluid will experience on the way to the loss zone is highly imperfect, the chances of failure due to incorrect gelation time are high. A fluid that responds to some “cue” or trigger that is unique to the appropriate wellbore location is therefore desirable.
Triggerable setting has been identified as the major technical barrier to success in previous attempts to apply polyurethane to major to catastrophic lost circulation to large natural fractures. A polyurethane fluid that begins to set only on a chemical cue provided at or near the loss zone stands the best chance of sweeping the drill mud efficiently and setting in the loss zone, and presents the least risk of premature setting. A solution to triggerable setting will allow oilfield engineers to simultaneously solve problems relating to physical placement (location of LCM in the natural fracture and effective displacement of mud already in the fracture) and setting time (start and rate of gelation) of a dynamically thickening polyurethane-based chemical LCM, giving a robust, reliable solution to this problem. In other words, a chemical LCM has to be delivered accurately to a loss-causing feature and designed to set only when it has been so delivered.
Conventional approaches to triggering have fallen into two broad categories: the mixture of the polyisocyanate (“A side”) and polyalcohol (“B side”) components at operating depth by means of a static mixer (see for example Mansure SPE74556) or other tool, or mixture of the A and B sides prior to injection with some chemical control over the reaction rate (see for example Spurlock SPE1237). The former method can be thought of as “grouting at depth” and the latter is operationally equivalent to cementing in that the LCM is chemically engineered to have a particular setting time based on the temperature profile it will experience during placement. References disclosed above are incorporated herein in their entirety, by reference thereto.
Water is a common potential contaminant in the flow path, wellbore, and formation. Water reacts with isocyanates to form CO2 gas and amines. This process is roughly one tenth as fast as the isocyanate-alcohol reaction that builds strength. However, amines react with isocyanates 10 to 100 times faster than alcohols to form urea linkages. Strict control over chemical kinetics when water is present is very difficult and although PU that is “allowed to solidify by the agency of water” has been described in the past (see for instance U.S. Pat. No. 3,623,330 to Nakade, incorporated by reference herein in its entirety) the technique as described is unreliable because water has a direct effect on control over setting time, which in turn affects reliability. Setting time and physical placement must be simultaneously controlled to deliver a good plug. Since it is almost impossible to completely guarantee a water-free flow path, any system that mixes isocyanates and polyols to begin the gelation process is susceptible to water contamination and is therefore not reliable. All of these problems are greatly exacerbated at elevated temperatures.
An alternative method for lost circulation is proposed herewith.